Welded joint between dissimilar metals



ariaasa Patented Nov. 13, 1956 ice WELDED JOINT BETWEEN DISSEMILAR METALS Otis R. Carpenter, Barberton, and Nicholas C. Jessen, Akron, Ohio, assignors to The Babcock 8.; Wilcox Company, Rockleigh, N. 3., a corporation of New Jersey No Drawing. Application June 15, 1950, Serial No. 168,385

3 Claims. (Cl. 29196.1)

This invention relates to the production of a Welded joint between austenitic and ferritic materials suitable for high temperature, high pressure service under conditions involving thermal shock and cyclic temperature and load applications.

Such conditions are encountered in high temperature process plants such as, for example, oil refineries, in vapor or steam generators, and in heat exchangers of various types. The particular problems in any one type of installation may differ in one or more aspects from those in another type. Thus, refineries involve high temperatures but only moderate pressures, in conjunction with alternating oxidation and reducing conditions, corrosive environments and the like. In steam generators a more complex stress condition exists due to the combined actions of high operating pressures and high operating temperatures, which are further aggravated by cyclic variations in these factors. While the invention is of general application under high temperature, high stress conditions in any type of installation, particular reference will be made, by way of example only, to the high temperature and high stress conditions encountered in steam generators.

In order to obtain higher efiiciencies, the outlet steam temperatures and the operating pressures of central station steam generators have been constantly increasing, and presently some central station steam generating units have outlet temperatures of 1050 F. and operating pressures of over 2000 p. s. i. The increasing use of such high temperatures and pressures has brought with it problems of providing materials and joints between such materials Which will successfully withstand the stresses encountered thereat.

The long time load carrying characteristics of metals at high temperatures, together with the economics involved, have led steam generator designers to use both aastenitic and ferritic materials for the outlet components of the steam generators. For example, both types of material may be used in the superheater and its supports, and in the main steam line from the generator to the turbine. Use of both types of materials in the same component requiresthat particular attention be given to the junctions between these materials, which junctions must operate under the particular temperature and stress conditions encountered in producing steam at relatively high temperatures. In a superheater, for example, the external surface and the superheater support lugs are at a higher temperature than the internal surface of the superheater tubes, due to the higher temperatures of the heating gases as compared to the temperature of the steam flowing through the superheater. The reverse is true with respect to the steam line leading to the turbine.

Operation under stress at such high temperatures introduces many problems due to the difierential expansion and contraction of the dissimilar materials on either side of the joint, their relative surface and structural stability, etc. Aside from mechanical stresses, such as, for example,

those due to differential thermal expansion and contraction, the factors influencing the service life of Welded joints between ferritic and austenitic materials have been basically of a metallurgical nature, such as carbon depletion in the heat affected zone of the ferritic material, notching due to oxide penetration occurring therein, micro-fissuring in the weld junction, and accelerated creep due to these conditions. Examples of joints between ferritic and austenitic materials, with which these problems are encountered, are the joining of a ferritic alloy having substantially 2 4% chromium to an austenitic alloy of the l88 or 25-20 type.

The present invention is particularly directed to the production of a welded joint, between austenitic and ferritic materials, which has the requisite strength, ductility, and oxidation resistance to assure satisfactory performance under conditions of high stresses occasioned by thermal shock, cyclic temperature and load application, and diflferential thermal expansion. It has been found that these objectives can be obtained by forming the welded joint by fusion deposition of a ferritic alloy metal having substantially the composition of the ferritic alloy workpiece excepting for carbon. The carbon content of the fusion deposited ferritic weld is maintained at a value much lower than that of the ferritic alloy workpiece, and does not exceed 0.05%.

For example, in weld uniting a ferritic alloy workpiece having a chromium content of substantially 2%% and 1% molybdenum to an austenitic alloy workpiece having, for example, 18 to 25% chromium, and 8 to 20% nickel, a satisfactory joint can be produced by utilizing a Welding rod having the same composition as the fcrritic workpiece, but with a carbon content of 0.05% or less. To compensate for the possible reduction in strength of the fusion deposited weld metal due to the low carbon content thereof, strengthening additives may be used. Typical strengthening additives are molybdenum, manganese, chromium, and/or nickel. Tungsten, vanadium, zirconium, columbium, or titanium may also be used advantageously. Silicon may be incorporated to enhance oxidation resistance.

Typical examples of the application of the invention are the welding of cast austenitic support lugs to ferritic superheater tubes and the butt welding of a ferritic high temperature-high pressure service tube to an austenitic high temperature-high pressure service tube. A typical example of a superheater support lug is shown in Gilg Patent No. 2,134,713. In the first example, the stress conditions are due not only to the difference in expansion characteristics but also to temperature differences between the superheater tube and its support lug, one carrying relatively lower temperature steam and the other being enveloped by relatively higher temperature heating gases, and to the cantilever loading of the tube and lug. In the butt welded tube assembly, stresses result from internal pressures, high temperatures, and ditferential thermal expansion.

When standard, commonly available ferritic electrodes of substantially the composition of the ferritic workpiece, but having a carbon content in excess of 0.05%, are used for welding austenitic alloy members to ferritic members, the dilution of the deposited weld metal with chromium and nickel may result in extremely hard and brittle microstructures, such as chromium carbides, for example, in the Weld deposit. Such extremely hard and brittle microstructures are conducive to micro-fissuring when the welded component is used at high temperatures and under conditions of high stress. Furthermore, such microstructures would comprise brittle regions which may serve as potential starting points for failure under the stresses due to substantial differential thermal expansion of the ferritic and austenitic workpiece. Failures have been known to occur in the ferritic workpiece at the carbon depleted region thereof, where the carbon has migrated toward the line of fusion of the weld joint, producing thereat a very hard structure. The carbon depleted area is correspondingly weakened.

The welded joint of the present invention being formed from a low carbon ferritic weld deposit, will not result in excessively hard and brittle microstructures in view of the low carbon level of the weld metal which would tend to prevent dilution with the austenitic base metal. As compared with a weld deposit of an austenitic material, such as 25-20 or 18-8 alloys, the low carbon ferritic weld deposit has the advantage that it is able to satisfactorily withstand the high stresses associated with high temperature operations, particularly under cyclically varying temperature and pressure conditions, to a far greater degree than an austenitic weld deposit.

A preferred composition of a weld rod for use in form ing the welded joint with the invention method includes carbon from 0.00 to 0.05% and one or more of the following:

Percent Manganese from 0.00 to 3.00 Silicon from 0.00 to 3.00 Chromium from 0.00 to 30.00 Molybdenum from 0.00 to 4.00 Nickel from 0.00 to 10.00

The composition may further include up to 4% of tungsten, vanadium, zirconium, columbium or titanium. The weld rod may be formed in any desired manner, either by providing an alloy of the foregoing materials, or by coating an iron rod with the additive alloying elements included in the coating. The proportions of the additive elements are selected in such manner that a predominantly ferritic structure is provided.

The actual welding may be performed by any of the processes known to those skilled in the art, such as shielded metal arc welding, atomic hydrogen welding, inert gas arc welding, submerged flux arc welding, or the like, depending upon the particular application involved.

Welds between austenitic and ferritic members made in accordance with the present invention have given satisfactory performances, under conditions of high temperature and under the high stresses involved in steam generation, particularly as compared with welds formed from austenitic weld deposits. In a particular example, specimens were prepared involving the welding of a 25 Crl2 Ni cast superheater lug to a 2%, chromiuml% molybdenum tube utilizing, in one case, a 25 Cr20 Ni electrode and in the other case the low carbon 2%% chromiuml% molybdenum electrode according to the present invention. After about 15 quenches from 1150 F. to 80 F., the lugs welded with the austenitic material separated from the tube, whereas those welded with the invention low carbon electrode showed no visible cracking or other signs of distress even after 100 such quenches.

The use of the low carbon weld deposit between a ferritic alloy, having 2%% chromium and up to 0.15 carbon, and an austenitic material, reduces carbon migration and thus reduces somewhat the tendency to failure under repeated cyclic stresses, due to such carbon depletion. Additionally, in such applications as the welding of austenitic support members to ferritic superheater tubes, the low carbon ferritic weld deposit removes the point of high stress due to expansion, from the colder face of the tube to a higher temperature zone having a lesser range of temperature cycling and consequently lower operating stresses. It improves the metallurgical transition between the austenitic alloy and the ferritic alloy, advantageously relieving the expansion stress problems by eliminating the sharp transition from a hard to a soft zone at the fusion line.

Typical examples of austenitic materials which have been successfully welded to ferritic materials by the present invention are alloys such as 18 Cr8 Ni, 25 Cr12 Ni, 19 Cr9 Ni, and 25 Cr20 Ni, the ferritic material being, by way of example, an alloy containing from 2 4% to 5% Cr and 1% Mo. In each case, the particular ferritic weld metal used to form the joint is selected to have an as-welded composition substantially the same as that of the ferritic workpiece except for the carbon content.

For high temperature and high pressure steam generating service, the low carbon ferritic weld metal of the invention must have sufficient strength to resist creep at elevated temperatures andthis can be attained by using sufl'icient strengthening elements such as manganese, molybdenum, etc., to replace the carbon not present and provide an alloy which is non-air-hardening. Also, the weld metal alloy should have sufiicient oxidation resistance at elevated temperatures to make it practicable for use in high temperature installations without the danger of formation of notches due to oxide penetration. This may be effected by adding silicon up to 4%, for example.

While specific examples of the invention have been described above, it should be understood that the invention principles may be otherwise embodied within the the scope of the invention.

We claim:

1. A welded joint for use under cyclically variable elevated temperatures and pressures, comprising a Cr-Ni austenitic alloy steel member, a low alloy ferritic steel member of the chromium containing type and having a carbon content in the range of 0.10% to 0.15%, and a weld deposit uniting said members and consisting of a ferritic alloy steel having substantially the same composition as said ferritic steel alloy member but containing carbon in an amount not exceeding 0.05

2. A welded joint as claimed in claim 1 in which said ferritic alloy steel weld deposit includes at least one of the following elements:

Percent Manganese not exceeding 3.00 Silicon not exceeding 3.00 Chromium not exceeding 30.00 Molybdenumnot exceeding 4.00 Nickel not exceeding 10.00

balance iron with the usual incidental impurities; the relative proportions of the additives being selected to produce a predominantly ferritic weld structure.

3. A welded joint for use under cyclically variable elevated temperatures and pressures, comprising a Cr-Ni austenitic alloy steel member, a ferritic alloy steel member containing approximately 2.25% chromium and having a carbon content of the order of 0.15%, and a weld deposit uniting said members and consisting of a ferritic alloy steel having substantially the same composition as said ferritic steel alloy member but containing carbon in an amount not exceeding 0.05

References Cited in the file of this patent UNITED STATES PATENTS (Other references on following page) 6 UNITED STATES PATENTS Welding Handbook, third ed., pp. 609, 640, 641, 650, 2,544,336 Linnert Man 6 1951' 651 and 670. Published by American Welding Society, 2,564,474 Field Aug. 14, 1951 New Ymk,

Welding Handbook, 1942 ed., pp. 773, 794 and 795.

OTHER REFERENCES 5 Published by the American Welding Society, 33 West 39th Procedure Handbook of Arc Welding Design and Prac- St, New York,

tice. Eighth ed., p. 398. Published by The Lincoln Elec- American Machinist, March 1, 1945, ,pp-

tric Co., Cleveland, Ohio. 

1. A WELDED JOINT FOR USE UNDER CYCLICALLY VARIABLE ELEVATED TEMPERATURES AND PRESSURES, COMPRISING A CR-NI AUSTENITIC ALLOY STEEL MEMBER, A LOW ALLOY FERRITIC STEEL MEMBER OF THE CHROMIUM CONTAINING TYPE AND HAVING A CARBON CONTENT IN THE RANGE OF 0.10% TO 0.15%, AND A WELD DEPOSIT UNITING SAID MEMBERS AND CONSISTING OF A FERRITIC ALLOY STEEL HAVING SUBSTANTIALLY THE SAME COMPOSITION AS SAID FERRITIC STEEL ALLOY MEMBER BUT CONTAINING CARBON IN AN AMOUNT NOT EXCEEDING 0.05%. 